ltfs_ordered_copy: preserve directory attributes (#392)

[MitchelTighe]

Fixes #392

ltfs_ordered_copy creates destination directories with os.mkdir() only, so even with -p or -a every copied directory gets the current time and the umask default mode, and its extended attributes are dropped.

Change

• While walking the source tree, remember each source/destination directory pair.
• After all files have been copied, apply the attributes with shutil.copystat(), plus the xattrs when -a is given.
  • This has to happen after the copy, because writing a file updates the mtime of the directory that contains it.
  • It runs deepest first, because updating a child directory updates its parent.
• Without -p/-a the behaviour is unchanged. A failure to copy one directory's attributes is logged as a warning and doesn't abort the copy.

Testing
I ran the tool against a small local harness, an on-disk source tree plus an xattr shim standing in for an LTFS volume, so no real tape was involved. These cases were checked:

• -r -p on nested directories with distinct modes and mtimes: before the patch they got umask/now; after it they match the source.
• An empty directory keeps its attributes.
• Without -p, directories keep the default mode, as before.

Written with help from an AI assistant (Claude).

Update (second commit): after a closer review, three things were tightened:

• A source given as dir/ or . copies its contents into DEST. In that case DEST's own attributes are now left alone; the first version overwrote them with the source directory's.
• Directories are now processed deepest-destination-first, so a parent made read-only can't block its children when there are several sources.
• Extended attributes are copied before copystat(), and an error from xattr.list() is logged as a warning instead of aborting the run.

Out of scope: intermediate directories created by --keep-tree (via os.makedirs()) still get default attributes. That's the same as current main, and it would be better handled separately.